(1) Field of the Invention
The present invention relates to an optical wavelength selection control system using a wavelength division multi-access technology in optical networks.
(2) Description of the Related Art
The optical networks using optical fibers for transmission paths are expected to be increasingly widely used in the future because of such advantages that the optical fiber offers broad bandwidths allowing to transmit a large quantity of data and that the optical fiber is free from any induced noise. A conventional system of such optical networks has been reported by M. Chen et al under the title "A Media-Access Protocol for Packet-Switched Wavelength Division Multiaccess Metropolitan Area Networks" in IEEE-Journal on Selected Areas in Communications, Vol. 8, No. 6, August 1990, pages 1048-1057.
FIG. 1 shows an architecture of the above conventional optical networks with three work stations (W/S). Electrical signals from the work stations 100-102 are respectively converted at optical interfaces (IF) 103-105 to optical packet signals of wavelengths .lambda.1, .lambda.2 and .lambda.3 which have respectively been fixed in advance, and the converted optical packet signals are sent out to an optical star coupler 106. At the optical star coupler 106, the optical packet signals from the optical interfaces 103-105 are wavelength-multiplexed, and the signals thus wavelength-multiplexed are sent back to the optical interfaces 103-105. At each of the optical interfaces 103-105, one optical packet signal of a predetermined wavelength is selected from among the optical signals in which the inputted packet signals with wavelengths .lambda.1, .lambda.2 and .lambda.3 are wavelength-multiplexed. The optical packet signals thus selected are converted to electrical signals which are sent out respectively to the work stations 100-102.
FIG. 2 shows details of one of the optical interfaces 103-105 used in the conventional optical networks shown in FIG. 1. The electrical packet signal from the work station concerned is inputted to a control circuit 200. The control circuit 200 sends a header portion of the electrical packet signal to a fixed wavelength light source 202 having a fixed wavelength of .lambda.c and a data portion to a fixed wavelength light source 201 having a fixed wavelength of .lambda.i which is unique to each work station. The fixed wavelength light source 202 transmits an optical header signal of the wavelength .lambda.c to the time slot which is unique to the work station, while the fixed wavelength light source 201 transmits an optical data signal of the wavelength .lambda.i, and these signals are transmitted to the star coupler 106 shown in FIG. 1 through an optical combiner 203. An optical beam splitter 204 transmits to an optical signal receiver 205 an optical signal of the wavelength .lambda.c in which a header portion from each of the work stations is time-division-multiplexed from the wavelength multiplexed signal received from the star coupler 106, and transmits to a tunable wavelength filter 206 an optical signal in which a data portion from each of the work stations is wavelength-multiplexed. The control circuit 200 receives from the optical signal receiver 205 the optical header signals from all the work stations, determines which station has transmitted the data portion to which station as a receiver, and controls the tunable wavelength filter 206 in such a way that it selects only the optical signal of the unique wavelength assigned in advance to a work station which has sent out the data designating the work station connected to that tunable wavelength filter. The output optical signal of the tunable wavelength filter 206 is converted to an electrical signal by an optical signal receiver 207 and is transmitted to the corresponding work station.
As above, each work station transmits a data portion of the packet signal at a predetermined wavelength, and the tunable wavelength filter selects the optical signal of the predetermined wavelength, and this enables optical signal communication between any work stations.
In the above conventional wavelength selection control system in optical networks using a wavelength division multiaccess technology, when a plurality of work stations send optical packet signals to the same work station simultaneously, there occurs a collision or conflict so that the work station is controlled to select and receive only one optical packet signal of a predetermined wavelength. Then, only the success or failure in the collision is made known to each work station, that is, in the form of an acknowledgement (ACK) signal and a not-acknowledgement (NACK) signal. Thus, there are possibilities for the stations having lost in the collision to resend the optical packet signals simultaneously thereby causing a collision to occur again, which leads to the lowering of the efficiency of the network system.